[PATCH 1/1] dma-direct: fix use of max_pfn

[PATCH 1/1] dma-direct: fix use of max_pfn

[Petr Tesarik]

Calculate the correct physical address of the last byte of memory. Since
max_pfn is in fact "the PFN of the first page after the highest system RAM
in physical address space", the highest address that might be used for a
DMA buffer is one byte below max_pfn << PAGE_SHIFT.

This fix is unlikely to make any difference in practice. It's just that the
current formula is slightly confusing.

Signed-off-by: Petr Tesarik <ptesarik@suse.com>
---
 kernel/dma/direct.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 8f43a930716d4..fefa6c4ac467f 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -39,7 +39,7 @@ static inline struct page *dma_direct_to_page(struct device *dev,
 
 u64 dma_direct_get_required_mask(struct device *dev)
 {
-	phys_addr_t phys = (phys_addr_t)(max_pfn - 1) << PAGE_SHIFT;
+	phys_addr_t phys = ((phys_addr_t)max_pfn << PAGE_SHIFT) - 1;
 	u64 max_dma = phys_to_dma_direct(dev, phys);
 
 	return (1ULL << (fls64(max_dma) - 1)) * 2 - 1;
@@ -540,7 +540,7 @@ int dma_direct_mmap(struct device *dev, struct vm_area_struct *vma,
 
 int dma_direct_supported(struct device *dev, u64 mask)
 {
-	u64 min_mask = (max_pfn - 1) << PAGE_SHIFT;
+	u64 min_mask = ((u64)max_pfn << PAGE_SHIFT) - 1;
 
 	/*
 	 * Because 32-bit DMA masks are so common we expect every architecture
-- 
2.53.0

Re: [PATCH 1/1] dma-direct: fix use of max_pfn

[Marek Szyprowski]

On 10.04.2026 13:35, Petr Tesarik wrote:
> Calculate the correct physical address of the last byte of memory. Since
> max_pfn is in fact "the PFN of the first page after the highest system RAM
> in physical address space", the highest address that might be used for a
> DMA buffer is one byte below max_pfn << PAGE_SHIFT.
>
> This fix is unlikely to make any difference in practice. It's just that the
> current formula is slightly confusing.
>
> Signed-off-by: Petr Tesarik <ptesarik@suse.com>

Applied to dma-mapping-fixes, thanks!

> ---
>  kernel/dma/direct.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 8f43a930716d4..fefa6c4ac467f 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -39,7 +39,7 @@ static inline struct page *dma_direct_to_page(struct device *dev,
>  
>  u64 dma_direct_get_required_mask(struct device *dev)
>  {
> -	phys_addr_t phys = (phys_addr_t)(max_pfn - 1) << PAGE_SHIFT;
> +	phys_addr_t phys = ((phys_addr_t)max_pfn << PAGE_SHIFT) - 1;
>  	u64 max_dma = phys_to_dma_direct(dev, phys);
>  
>  	return (1ULL << (fls64(max_dma) - 1)) * 2 - 1;
> @@ -540,7 +540,7 @@ int dma_direct_mmap(struct device *dev, struct vm_area_struct *vma,
>  
>  int dma_direct_supported(struct device *dev, u64 mask)
>  {
> -	u64 min_mask = (max_pfn - 1) << PAGE_SHIFT;
> +	u64 min_mask = ((u64)max_pfn << PAGE_SHIFT) - 1;
>  
>  	/*
>  	 * Because 32-bit DMA masks are so common we expect every architecture

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland